The present invention relates generally to electronic chips and devices, and more particularly, to a method and device suitable for use in performing DC parametric tests.
Recent years have seen a rapidly increasing demand for highly integrated mixed-signal integrated circuits (IC""s). This demand is mostly driven by the ever-expanding communications industry. However, as the level of integration increases, more and more mixed-signal components are becoming buried deep inside large amounts of digital circuitry without any external I/O access. This creates a difficult problem for initial device and circuit characterisation and diagnosis, as well as during a production test. For example, to measure the bias current for a high precision ADC circuit requires some form of external access. However, the access mechanism, such as a test bus, can introduce additional noise from off-chip sources.
Typically circuit characterisation includes the determination of the electrical characteristics of a circuit such as for example measuring the input/output impedance of an amplifier circuit, or finding the voltage transfer characteristics of an amplifier circuit or transistor device amongst others.
One particular area of IC testing that is being affected is the DC parametric tests. These tests are typically conducted to characterise a wide variety of mixed-signal circuits such as Analog-to-Digital Converters (ADCs), PLLs and bias networks. Also, these tests are used in digital test applications such as pad current leakage and IDDQ tests. For example, the pad current leakage test and the IDDQ test are common test techniques for detecting faults in digital ICs.
DC parametric tests are generally classified as one of two types. The first type of DC parametric test involves forcing a voltage at a circuit node while measuring the current that flows into the node. Commonly used method for on-chip current measurements include using device having either a transimpedance amplifier, as shown in FIG. 1a), an integrating network as shows in FIG. 1b) or a shunt resistance, as shown in FIG. 1c). For additional information regarding the above mentioned methods, the reader is invited to refer to the following documents:
1. Teradyne, Inc., xe2x80x9cLow Current Ammeter Channel Cardxe2x80x9d, Advanced Mixed-Signal Instrumentation Manual, 1996.
2. C. D. Thompson, S. R. Bernadas, xe2x80x9cA Digitally-Corrected 20b Delta-Sigma Modulatorxe2x80x9d, Proc. IEEE International Solid-State Circuits Conference, pp. 194-195, 1994.
3. U.S. Pat. No. 5,274,375 issued to Charles D. Thompson Dec. 28, 1993;
4. M. Breten, T. Lehmann, E. Bruun, xe2x80x9cIntegrating Data Converters for Picoampere Currents from Electrochemical Transducersxe2x80x9d, Proc. IEEE International Symposium on Circuits and Systems, Vol. 5, pp. 709-712, May 2000.
5. C. B. Wang, J. Todsen, T. Kalthoff, xe2x80x9cA Dual Channel 20 Bit Current-Input A/D Converter for Photo-Sensor Applicationsxe2x80x9d, Proc. Southwest Symposium on Mixed-Signal Design, pp. 57-60, 2000.
6. Burr-Brown Product #DDC 112
7. J. Kotowski, B. McIntyre, J. Parry, xe2x80x9cCurrent Sensor IC Provides 9 bit+ Sign Result without External Sense Resistorxe2x80x9d, Proc. IEEE Custom Integrated Circuits Conference, pp. 35-38, 1998;
8. U.S. Pat. No. 5,867,054 issued to Jeffrey P. Kotowski Feb. 2, 1999;
9. National Semiconductor Product #LM3814
The contents of the above documents are hereby incorporated by reference.
A deficiency of devices of the type described above is that they involve the use of elaborate Analog-to-Digital Converters (ADCs) with trimmed components, which makes these devices expensive and relatively non-scalable for on-chip implementation. Another deficiency of devices of the type described above is that they make use of op-amps (operational amplifiers) which also makes them relatively non-scalable for on-chip implementation. Generally, the size of the op-amp circuit does not shrink to the same extent as the size of logic circuits do as IC technology advances.
The second type of DC parametric test involves forcing a known current into a circuit node while measuring the voltage at the node.
A deficiency of commonly used on-chip current sources is that they generally suffer from low output resistance and shifts in current levels due to process variation. Such current sources are described in W. Sansen et al., xe2x80x9cA CMOS Temperature-Compensated Current Referencexe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. 23, pp. 821-824, June 1988 and in H. J. Oguey et al., xe2x80x9cCMOS Current Reference Without Resistancexe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. 32, pp. 1132-1135, July 1997 whose contents are herein incorporated by reference. Other on-chip current source implementations, of the type described in Burr-Brown Corporation, xe2x80x9cDual Current Source/Current Sinkxe2x80x9d, REF200 (Datasheet), October 1993 and in U.S. Pat. No. 4,792,748 issued to David M. Thomas et al. in Dec. 20, 1998, can generally achieve good current accuracy but require laser-trimmed on-chip resistors, which is costly when multiple measurement units are required on a single chip. The contents of the above documents are hereby incorporated by reference.
In the context of the above, there is a need in the industry to provide a method and device for use in performing DC parametric tests that alleviates at least in part problems associated with the existing devices and methods.
In accordance with a first broad aspect, the invention provides a circuit device suitable for use in performing a DC parametric test on an external load. The circuit device includes an input suitable for receiving a forcing parameter signal, an output suitable for releasing to the external load a signal approximating the forcing parameter signal, a first circuit segment and a second circuit segment. The first circuit segment is located between the input and the output and includes a search entity, an intermediate voltage point and an internal load between the intermediate voltage point and the output of the circuit device. The second circuit segment is connected in a feedback arrangement with the first circuit segment. The second circuit segment provides the search entity in the first circuit segment with a first voltage signal indicative of the voltage at the output of the circuit device. The search entity is adapted for generating a second voltage signal on the basis of the forcing parameter signal and the first voltage signal received from the second segment and for applying the second voltage signal to the intermediate voltage point. The application of the second voltage signal to the intermediate voltage point causes a change in either one of the voltage signal or the current signal at the output of the circuit device such that, at equilibrium, a signal approximating the forcing parameter signal is caused at the output.
In accordance with another broad aspect, the invention provides a circuit for performing a DC parameter test on an external load. The circuit comprises a circuit input, a circuit output, a search unit, a circuit module having digital-to-analog conversion functionality and load functionality and an analog-to-digital converter (ADC). The circuit input is for receiving a forcing parameter signal. The circuit output is for connection to the external load. The search unit has a first input connected to the circuit input and has a second input and an output. The circuit module has digital-to-analog conversion functionality and load functionality and is connected between the output of the search unit and the circuit output. The analog-to-digital converter (ADC) is connected between the circuit output and the second input of the search unit. The search unit is adapted to generate a digital target voltage at its output on the basis of a voltage at the circuit output and the forcing parameter signal whereby a signal approximating the forcing parameter signal is derived at the circuit output.
Advantageously, the above-described circuit can be implemented using some digital logic as permitted by the use of ADCs and DACs. The digital logic allows taking advantage of the down-scaling of digital integrated circuit technology and facilitates the on-chip implementation of such devices.
In accordance with a specific example implementation, the forcing parameter signal is a forced voltage signal.
In accordance with an alternative specific example implementation, the forcing parameter signal is a forced current signal.
Specific examples of implementation may make use of a forcing parameter signal that is in either one of an analog format or digital format.
In a specific example of implementation, the circuit module having digital-to-analog conversion functionality and load functionality includes a low-pass filter module one side of which is connected to the circuit output and a pulse generator module connected between the output of the search unit and another side of the low-pass filter module.
In an alternate specific example of implementation, the circuit module having digital-to-analog conversion functionality and load functionality includes an internal load one side of which is connected to the circuit output and a digital-to-analog converter (DAC) connected between the output of the search unit and another side of the internal load.
In accordance with a first non-limiting implementation, the internal load in the circuit device is a linear non-inverting load. Such a linear non-inverting load may include one or more linear resistor elements, RC (resistor/capacitor) circuit elements and any other suitable linear analog circuit having linear non-inverting properties. Such linear analog circuits having linear non-inverting properties may include non-linear components arranged in such a manner to produce a linear non-inverting load. Such non-linear components may include for example PMOS circuits, NMOS circuits, CMOS circuits, BJT circuits, BiCMOS circuits, JFET circuits and MESFET circuits.
In accordance with a second non-limiting implementation, the internal load in the circuit device is a non-linear and inverting load. Such a non-linear and inverting load may include one or more MOS elements, BJT circuits, JFET circuits, diode circuits, MESFET or BiCMOS circuits. MOS configurations including a CMOS circuit, a PMOS circuit and NMOS circuit may also be used.
In accordance with a third non-limiting implementation, the internal load in the circuit device is a non-linear and non-inverting load. Such a non-linear and non-inverting load may include one or more MOS elements, BJT circuits, JFET circuits, diode circuits, MESFET or BiCMOS circuits.
In accordance with a fourth non-limiting implementation, the internal load in the circuit device is a linear and inverting load. Such a linear and inverting load may include one or more MOS elements, BJT circuits, JFET circuits, MESFET or BiCMOS circuits.
Advantageously, by using a non-linear internal load, larger changes in current at the output can be established for a smaller corresponding change in voltage applied at the output. In addition, the downscaling of digital circuit will limit the power supply to lower voltage levels, which in turn will limit the output voltage range of analog circuits. Consequently, circuits of the type described above making use of a non-linear internal load will be affected to a lesser extent by reductions in power supply voltages.
In a specific non-limiting example of implementation where the forcing parameter signal is a forced voltage signal, the search unit includes a digital comparator, a digital integrator module and an output. The digital comparator is for generating a digital difference voltage signal dependent on the difference between the forced voltage signal and the digital approximation of the voltage signal at the output of the circuit device. The digital integrator module is adapted for processing the digital difference voltage signal to derive the digital target voltage. The digital target voltage is released at the output of the search unit for processing by the digital-to-analog converter module. Where the forcing parameter signal is an analog signal, the search unit further includes an analogue-to-digital converter module for processing the forced voltage signal to generate a corresponding digital forced voltage signal. The digital forced voltage signal is then provided to the digital comparator.
In accordance with a specific example of implementation, the analog-to-digital converter module in the circuit device includes an analog comparator, a digital integrator and a feedback circuit. The analog comparator receives a signal indicative of the voltage at the output and a tracking voltage and generates a difference signal on the basis of the signals received. The digital integrator receives the difference signal and generates successive digital approximations of the voltage signal at the output of the circuit device. The feedback circuit processes the successive digital approximations of the voltage signal to generate the tracking voltage and provide the tracking voltage to the analog comparator. In a non-limiting implementation, the feedback circuit includes a digital-to-analog converter module. In accordance with an alternative specific example of implementation, the digital integrator in the analog-to-digital converter module is replaced by a successive-approximation circuit (SAR) module. For additional information regarding successive-approximation circuits (SAR), the reader is invited to consult D. A. Johns, K. Martin, Analog Integrated Circuit Design, John Wiley and Sons, Inc., pp. 492-493, 1997. The content of this document is hereby incorporated by reference.
In accordance with another broad aspect, the invention provides a system for providing a current measurement suitable for use in performing a DC parametric test on an external load. The system includes a voltage generating circuit device and a current measurement circuit. The voltage generating circuit device includes an input for receiving a signal indicative of a forced voltage, an output suitable for releasing to the external load a signal approximating the forced voltage, a first circuit segment and a second circuit segment. The first circuit segment is between the input and the output and includes a search unit, an intermediate voltage point and an internal load between the intermediate voltage point and the output. The second circuit segment is connected in a feedback arrangement with the first circuit segment. The second circuit segment provides the search unit in the first circuit segment with a first voltage signal indicative of the voltage at the output. The search unit is adapted for generating a second voltage signal on the basis of the signal indicative of the forced voltage and the first voltage signal received from the second segment and applying the second voltage signal to the intermediate voltage point. The application of the second voltage to the intermediate voltage point causes a change in either one of the voltage signal or the current signal at the output such that, at equilibrium, a voltage approximating the forced voltage is caused at the output. The current measurement circuit includes a first input for receiving a first signal derived from the second voltage signal, a second input for receiving a second signal derived from the signal indicative of the forced voltage signal, search logic and an output. The search logic derives a certain current measurement on the basis of the first signal and the second signal. The certain current measurement is released at the output of the current measurement circuit. When the system is in equilibrium, the certain current measurement is indicative of an approximation of measurement of the current flowing at the output of the voltage generating circuit when the signal indicative of the forced voltage is applied to the output of the voltage generating circuit.
In accordance with a non-limiting example, the first signal derived from the second voltage signal includes a digital representation of the second voltage signal and the second signal derived from the signal indicative of the forced voltage includes a digital approximation of the signal indicative of the forced voltage.
In accordance with another broad aspect, the invention provides a system for providing a current measurement for use in performing a DC parametric test on an external load. The system includes a voltage generating circuit device and a current measurement circuit. The voltage generating circuit device includes a circuit input, a circuit output, a search unit, a circuit module having digital-to-analog conversion functionality and load functionality and an analog-to-digital converter (ADC). The circuit input is for receiving a signal indicative of a forced voltage. The circuit output is for connection to the external load. The search unit has a first input connected to the circuit input and has a second input and an output. The circuit module having digital-to-analog conversion functionality and load functionality is connected between the output of the search unit and the circuit output. The analog-to-digital converter (ADC) is connected between the circuit output and the second input of the search unit. The search unit is adapted to generate a digital target voltage at its output on the basis of a voltage at the circuit output and the signal indicative of the forced voltage whereby a signal approximating the forced voltage is applied at the circuit output. The current measurement circuit includes a first input for receiving a first signal derived from the digital target voltage, a second input for receiving a second signal derived from the forced voltage signal and a search logic module. The search logic module is coupled to the first and second inputs and derives a certain current measurement on the basis of the first signal derived from the digital target voltage and the second signal derived from the forced voltage signal and a search logic module. When the system is in equilibrium, the certain current measurement is indicative of an approximation of measurement of the current flowing at the output of the voltage generating circuit when the signal approximating the forced voltage is applied to the output of the voltage generating circuit. The certain current measurement is released at an output of the current measurement circuit.
In a specific implementation, the circuit module having digital-to-analog conversion functionality and load functionality includes a low-pass filter module one side of which is connected to the circuit output and a pulse generator module connected between the output of the search unit and another side of the low-pass filter module.
In an alternative specific implementation, the circuit module having digital-to-analog conversion functionality and load functionality includes an internal load one side of which is connected to the circuit output and a digital-to-analog converter (DAC) connected between the output of the search unit and another side of the internal load.
In accordance with a specific implementation, the search logic of the current measurement circuit includes a data structure having a plurality of entries, each entry providing a mapping between a data element conveying a given target voltage and data element conveying a given the forced voltage to a corresponding current measurement. The data structure may be stored on any suitable memory unit such as a RAM, ROM, PROM, EPROM and EEPROM. In a specific non-limiting implementation, the data structure is stored on a RAM device.
Advantageously, the search logic captures the DC transfer characteristic of the internal load at different current levels. Once known, the transfer characteristic of the internal load can be used indirectly to determine the value of an unknown current level at the output of the circuit device when the output is connect to an external load.
In accordance with yet another broad aspect, the invention provides a current generating circuit device suitable for use in performing a DC parametric test on an external load. The circuit device includes a circuit input, a circuit output, a search unit, a circuit module having digital-to-analog conversion functionality and load functionality and an analog-to-digital converter (ADC). The circuit input is for receiving a signal indicative of a forced current. The circuit output is for connection to the external load. The search unit has a first input connected to the circuit input and has a second input and an output. The circuit module having digital-to-analog conversion functionality and load functionality is connected between the output of the search unit and the circuit output. The analog-to-digital converter (ADC) is connected between the circuit output and the second input of the search unit. The search unit is adapted to generate a digital target voltage at its output on the basis of a voltage at the circuit output and the signal indicative of a forced current whereby a signal approximating the signal indicative of a forced current is derived at the circuit output.
In accordance with a specific example, the circuit module having digital-to-analog conversion functionality and load functionality includes a low-pass filter module one side of which is connected to the circuit output and a pulse generator module connected between the output of the search unit and another side of the low-pass filter module.
In accordance with an alternate example, the circuit module having digital-to-analog conversion functionality and load functionality includes an internal load one side of which is connected to the circuit output and a digital-to-analog converter (DAC) connected between the output of the search unit and another side of the internal load.
In accordance with a specific example of implementation, the search unit includes a data structure having a plurality of entries, each entry providing a mapping between:
1. a data element conveying the voltage signal at the output of the circuit device; and
2. a data element conveying a given forced current and a data element conveying a target voltage.
In a non-limiting implementation, the data structure may be stored on any suitable memory unit such as, but not limited to, a RAM, ROM, PROM, EPROM and EEPROM. In a specific embodiment, the memory unit is stored on a RAM.
In accordance with an alternative implementation, the search unit provides a data structure having a plurality of entries, each entry providing a mapping between:
1. data elements conveying voltage signals at the output and a data element conveying a given forced current signal; and
2. data elements conveying target voltage values.
These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings.